Sir Bart is one of the knights seen at the Jousting Tournament near the end of the film. He seems to have some authority, as he is able to halt the tournament when a boy constantly called Wart pulls the sword from the stone.
The other knights initially try to force Wart to repeat the feat. However, Kay and several other knights push him aside, under the mistaken belief that anyone can remove the sword a second time. However, both Sir Bart and Sir Pellinore stand up on behalf of Wart and allow the boy to successfully remove the sword, which reveals him as the rightful king of England.
Sir Bart asks Sir Ector, Wart's foster father, for the boy's name. After learning that Wart's proper name is Arthur, he leads the other knights into cheering on for Arthur as their new king.